INTRODUCTION 



The rapid corrosion of heat exchangers in air conditioning and 

 refrigeration equipment, especially in tropical marine areas, is a major 

 maintenance and energy use problem. In a brief survey conducted by the 

 Civil Engineering Laboratory (CEL) (Ref 1), it was learned that at Subic 

 Bay 90% to 95% of all air conditioning and refrigeration units have 

 liquid-to-air heat exchangers; at Guam and Diego Garcia, nearly 100% of 

 these units are used because of a water scarcity. Corrosion at Subic 

 Bay is not severe because the prevailing wind is over land and because 

 rainfall is high. Corrosion is severe at Guam and extremely severe at 

 Diego Garcia. For example, uncoated copper tube/aluminum fin heat 

 exchangers last less than 1 year at some Diego Garcia locations. In 

 contrast, aluminum fin condenser coils located within 1 mile of the 

 ocean at San Diego have a life expectancy of 5 years and a minimum life 

 of from 2 to 3 years. Heat exchangers are periodically washed at Subic 

 Bay, and this practice has started at San Diego, but is not done at Guam 

 or at Diego Garcia. The reasons for not doing this at the latter two 

 locations may be high labor costs or a general lack of maintenance 

 procedures, as well as a scarcity of water. 



Protective coatings, which are applied to metal surfaces exposed to 

 aggressive environments, have not been widely used on heat exchangers 

 because of a consequent initial decrease in heat transfer. However, Lee 

 (Ref 2) reported that the application of a thin protective coating on 

 gas-to-gas, liquid-to-gas, and liquid-to-liquid heat exchangers has only 

 a small effect on heat transfer. Furthermore, in a 28-day field trial 

 on steel pipe, liquid-to-liquid heat exchangers, Lee found that the heat 

 transfer coefficients of the coated heat exchangers remained constant. 

 The coefficients of the uncoated exchangers decreased to values below 

 those of the coated units within the trial period. Oldberg (Ref 3) 

 concluded from his study of compact military heat exchangers that the 

 standard brazed aluminum construction must have an organic or equally 

 resistive coating for protection against high salt atmospheres which 

 exist in many areas of the world. 



In a second phase of his work, Oldberg (Ref 4) reported that an 

 all-aluminum exchanger (Type 3003 tube and Type 7072 fins) showed less 

 corrosive attack than did the copper tube/aluminum fin units ; and phenolic 

 coatings of approximately 0.001 inch (1.0 mil, 0.0025 cm) thickness 

 considerably improved the corrosion resistance of the all-aluminum unit, 

 which looked good after 31 months of exposure in the Canal Zone. 



At Patrick Air Force Base (Ref 5), where the air contains hydrogen 

 sulfide and salt spray, the service life of uncoated all-copper heat 

 exchangers is 6 to 7 years; when these units are coated with 3 mils of a 



